Plaster of Paris casts have been used to immobilize body members for some time. These bandages are made by depositing plaster of Paris on a reinforcing scrim material such as gauze. When the plaster of Paris is dipped in water, the alphahemihydrate of calcium sulfate is converted to calcium sulfate dihydrate, which results in the hardening of the cast. Plaster of Paris casts, however, suffer from a number of disadvantages. X-ray transmission through the cast to determine whether a fracture has been properly set is extremely difficult. In addition, the cast is quite heavy and restricts the mobility of the patient wearing the cast. The casts are also very sensitive to water and may seriously lose their load-bearing capability if they become wet. In addition, the air permeability of the plaster of Paris cast is very limited, and, as a result, they do not allow evaporation of moisture from the skin beneath the cast, which may result in skin irritation beneath the cast.
In order to overcome the disadvantages of plaster of Paris casts, numerous attempts have been made to develop plastic or plastic-reinforced materials as a replacement for plaster of Paris.
U.S. Pat. Nos. 3,241,501 and 3,881,473 disclose casts which are made with a flexible fabric impregnated with a polymer which is capable of being cured by ultraviolet light. Although this casting material overcomes some of the disadvantages of plaster of Paris cast material, it requires a different technique in its application and also requires the use of an ultraviolet light source in order to cure the cast. These casts also require significantly longer times for the cast to set before they will be load bearing.
More recent attempts to produce substitutes for plaster of Paris include the polyurethane polymers disclosed in German Offenlegenschrift Nos. 2353212 and 2357931, U.K. Pat. No. 1,578,895, and PCT Application No. W081/00671. These bandages are open-weave fabrics coated with polyurethane prepolymers, that is, reaction products of isocyanates and polyols. The bandages are dipped into water in the same manner as the plaster of Paris and then applied to the limb of a patient. The water causes the prepolymer to polymerize and form a rigid polymer structure. In order to obtain the desired rapid hardening or setting of the bandage, it is necessary to have a catalyst system incorporated in the prepolymer formulation. The casting material disclosed in U.K. Pat. No. 1,578,895 employs amino polyols as catalysts and as the polyol components. The casting material disclosed in W081/00671 employs dimethylethanolamine (DMEA) or a mixture of DMEA and bis(2-dimethylaminoethyl) ether. These catalyst systems provide acceptable hardening of the prepolymers by catalyzing the water-isocyanate reaction. However, the presence of these catalysts in the prepolymer system also causes side reactions which gel the prepolymer in the bandage package. These side reactions are generally branching reactions resulting in biuret and allophanate formation and some formation of isocyanate trimer. The gelatin caused by the side reactions causes premature hardening or setting of the bandage in the package and, therefore, poor shelf life or shelf stability. The lack of adequate shelf stability can cause numerous difficulties in attempting to form a cast from such cast bandages. In order for the bandages to have an acceptable set time, it is necessary to adjust the polyurethane prepolymer components to the extent that the reaction with water is such that the set time of the finished bandage is satisfactory. The set time is the time after the bandage is dipped in water to the point where the cast made from the bandage is rigid and the limb of the patient is immobilized. In order to obtain acceptable set times, the polyurethane prepolymer bandages of prior art products had a limited shelf life, i.e., less than 12 months, which is not practical commercially.
Although there are numerous catalysts available to catalyze the water-isocyanate reaction of the prepolymer, these catalysts are not necessarily suitable for use in a cast bandage, as these catalysts do not provide adequate shelf life for the cast bandage. The particular catalyst employed in the present invention has previously been employed in the formation of polyurethane foams (see U.S. Pat. No. 3,645,925) and reaction injection molding elastomers (see U.S. Pat. No. 4,273,885).